The present invention relates to a method of preparing fine grain WC--Co(Ni)-powders for use in the manufacture of a cemented carbide.
Cemented carbide and titanium-based carbonitride alloys (often referred to as cermets) contain hard constituents based on carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a binder phase essentially based on Co and/or Ni. They are made by powder metallurgical methods of milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering.
The milling operation is an intensive grinding in mills of different sizes with the aid of milling bodies which are usually made of a cemented carbide. The milling time is of the order of several hours up to days. Milling is believed to be necessary in order to obtain a uniform distribution of the binder phase in the milled mixture. It is further believed that the intensive milling increases the reactivity of the mixture which further promotes the formation of a dense structure.
GB 346,473 discloses a method of making cemented carbide bodies. Instead of milling, the hard constituent grains are coated with the binder phase by an electrolytic method, pressed and sintered to a dense structure. This and other similar methods are, however, not suited for cemented carbide production in a large industrial scale and milling is almost exclusively used within the cemented carbide industry today.
However, milling has its disadvantages. Because of the long milling time, the milling bodies wear and contaminate the milled mixture which has to be compensated for. The milling bodies can also break during milling and remain in the structure of the sintered bodies. Furthermore, even after an extended milling, a non-homogenous rather than ideal homogeneous mixture may be obtained. In order to ensure an even distribution of the binder phase in the sintered structure, sintering has to be performed at a higher temperature than the theoretical.
An alternative way is to start from an intimate mixture of cobalt and tungsten, which mixture subsequently is carburized. U.S. Pat. No. 3,440,035 discloses such a method of preparing cemented carbide powder characterized in that an aqueous solution of ammoniumparatungstate (APT) and nitric or hydrochloric aqueous solution of, e.g., cobalt are mixed. The mixture is then subjected to a neutralizing reaction with ammonium hydroxide at a temperature of 20.degree. C. to 80.degree. C. when the pH-value of the mother solution after the reaction is adjusted to be between 4.5 to 8. The resultant fine composite precipitate containing tungsten and cobalt in the desired composition controlled by the reaction conditions is filtered, dried by heating and then subjected to reduction and carburization to obtain a WC--Co-composite powder in which the WC grain size generally is submicron.